1. Field of the Invention
The present invention relates to a method and apparatus for estimating whether an uneven wear is occurring in an edge of the tire shoulder from output signals of an acceleration sensor disposed on an inner surface side of the tire tread.
2. Description of the Related Art
A tire gets worn in the tread surface as it runs from having friction with the road surface. From the viewpoint of tire performance, it is desirable that the tread surface wears away evenly, thus causing no significant change in the shape of the contact patch (footprint). However, at the time of cornering, acceleration, or deceleration, lateral or fore-aft forces in relation to the traveling direction of the vehicle may work on tires. As a result, there occur differences in the amount of wear between the central part and the shoulder parts of the tire tread.
As these uneven wears accumulate, a tire may have an extremely worn edge of the tire shoulder. The tire with an advanced uneven wear like this is called an unevenly worn tire in contrast to an evenly worn tire which has an evenly worn surface of the tread.
A tire having an extreme uneven wear, if it keeps on being used continuously, may eventually fail to deliver its primary performance. Especially with winter tires, a problem that frequently arises is a lost grip on the road surface.
Also, an unevenly worn tire has a shape of the contact patch that is deviant from ideal. Hence, such a tire will experience a greater loss of fuel economy performance than a normally worn tire having an evenly worn tread surface.
There has been a method known in the art in which an acceleration sensor is disposed at each of the axial center and shoulder portions on an inner surface side of the tire tread. And using the output signals from these acceleration sensors, estimation is made whether an uneven wear is occurring in an edge of the tire shoulder (see Patent Document 1, for instance). More specifically, a differentiated acceleration waveform at the axial center and differentiated acceleration waveforms at the shoulder portions are obtained by differentiating each of the radial acceleration waveforms at the axial center and the shoulders detected by the respective acceleration sensors. Then a differentiated peak value ratio is calculated, which is a ratio between the differentiated peak value at the center and the differentiated peak value at the shoulder, both being the peak values at tire contact ends calculated from the respective differentiated acceleration waveforms. Now the calculated differentiated peak value ratio is compared with the differentiated peak value ratio having been calculated in advance of a tire without the presence of uneven wear in an edge of the shoulder portion to determine whether an uneven wear is occurring in an edge of the tire shoulder.
As described above, according to Patent Document 1, the uneven wear in the tire shoulder is estimated using the ratio between the differentiated peak value at the center and that at the shoulder calculated respectively from the output signals of the acceleration sensors disposed at the axial center and the shoulder portions on an inner surface side of the tire tread. Thus drops in accuracy in estimating an uneven wear due to changes in the traveling speed of the vehicle can be avoided.